Semiconductor optoelectronic devices such as light-emitting diodes (LEDs) and laser diodes (LDs) have a wide range of applications including use as indicator light components, as solid-state light sources and in optical storage systems. The suitability of a device for an application depends on the wavelength of the light generated by the device.
The wavelength of light emitted from an optoelectronic device depends on the properties of the light-emitting region of the device, which is described hereafter as the “active region” of the device. Semiconductor light-emitting devices are typically fabricated using an epitaxial deposition technique such as molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOVPE). In these epitaxial deposition techniques the layers of the device are deposited sequentially onto a substrate and this substrate is subsequently divided into individual chips each containing an optoelectronic device. A feature of these epitaxial deposition techniques is that there is usually little or no control of the properties of each deposited layer as a function of position in the plane of the substrate surface on a length scale comparable with the final size of the chips. Therefore, each chip has an active region that has essentially the same properties over its entire surface area. As a consequence of this, each device (chip) emits light with a single wavelength, or sometimes with a small continuous range of wavelengths distributed about a wavelength at which the output intensity has a maximum value. This type of device is not optimum for the many applications for semiconductor optoelectronic devices which require mixing of light with more than one wavelength. For example, to generate light emission which is perceived as white light requires mixing of light with at least two wavelengths. Therefore, there is a demand to fabricate optoelectronic device chips which emit light at more than one emission wavelength, and which are referred to hereafter as “polychromatic” light-emitting devices.
One approach to fabricating polychromatic light-emitting devices is to combine two or more discrete chips, with different emission wavelengths from one another, together in a single package. However, there are significant disadvantages associated with this multiple-chip approach. The need to combine more than one chip in the package causes the minimum achievable package size to be larger than is possible for a single-chip package. Furthermore, since the different wavelengths of light are emitted from separate locations, complex, expensive and bulky packaging can be required to ensure that there is effective mixing of the component wavelengths. Therefore, multiple-chip packages have disadvantages for applications requiring low-cost and compact light sources.
These disadvantages can be overcome by the use of monolithic polychromatic light emitters, where multiple-wavelength emission is delivered from a single chip. This type of device can be fabricated if active regions with different emission wavelengths are positioned close to one-another on a substrate such that, when the substrate is divided into discrete chips, each chip contains active regions with the different emission wavelengths.
Approaches exist in the prior art to deliver monolithic polychromatic light emitters using epitaxial deposition techniques. One approach is to vertically stack, along the deposition direction, two or more light-emitting device structures, each with a different emission wavelength. For example, a device formed from a light-emitting device fabricated from a II-VI semiconductor material deposited on top of a light-emitting device fabricated from a III-V semiconductor material is described in U.S. patent application No. 2002/0041148. Devices based around a similar vertical-stacking paradigm are also disclosed in U.S. patent application Nos. 2005/0067627 and 2006/0027820. The use of wafer bonding techniques to vertically join two light emitting devices which were deposited independently on different substrates is disclosed in U.S. patent application No. 2003/0047742. Disadvantages of these vertical stacking approaches include the complexity and high costs of vertically stacking devices with different emission wavelengths, the significant complexity of device processing which is required to enable independent control of the current injected into the devices with different emission wavelengths during operation, and the necessity to grow each active region with an independent emission wavelength individually.
A second approach to fabricating monolithic polychromatic emitters is to position active regions having different emission wavelengths laterally in the plane of the substrate. U.S. patent application No. 2005/0161683 describes growth of a first light emitting device on a substrate, the subsequent removal of this device from regions of the wafer by etching, and re-growth of a second device with a different emission wavelength in these etched regions. The resultant device therefore contains two independent light emitting devices positioned side-by-side on the original substrate. A similar result is achieved according to U.S. Pat. No. 6,681,064 by applying multiple epitaxial deposition steps in defined regions of a substrate to grow multiple light emitting devices on a single substrate. A significant disadvantage of these approaches is that although only one substrate is used, epitaxial growth of an entire device structure is required for each individual emission wavelength.
One well-known type of active region for light emitting devices contains quantum dots. A quantum dot is a potential box in which either electrons or holes, or both electrons and holes (hereafter electrons and holes are referred to collectively as “carriers”) are confined and experience significant quantum size effects in all three spatial dimensions owing to the small size of the box. A consequence of the three-dimensional quantum confinement is that the energy levels of the carriers are quantized into discrete levels and the density of states for carriers takes the form of one or more delta-functions.
In a semiconductor light-emitting device the active region of which contains quantum dots, the quantum dots are crystalline inclusions of a first semiconductor which are embedded in a matrix of a second semiconductor which has a wider forbidden energy band gap than the first semiconductor. The inclusions are sufficiently small in all three dimensions that carriers experience significant quantum size effects. The properties of quantum dots are strongly dependent on their size, composition and shape. Of particular relevance for active regions of light emitting devices containing quantum dots is that the wavelength of light emitted from the quantum dots can be strongly dependent on the size, composition and shape of the quantum dots.
A common method of preparing quantum dots for use in the active region of a light-emitting device is to exploit the self-organised formation of islands on a substrate surface during epitaxial deposition, such as by MBE or MOVPE techniques. Quantum dots suitable for active regions are formed when a capping layer is grown on top of the islands such that the island material becomes embedded between the capping material and the substrate.
When the forbidden energy band gap of the island material is smaller than that of the capping material and that of the material at the surface of the substrate on which the islands formed, the island material behaves as a potential box. Provided the dimensions of this box are small enough in all three spatial dimensions, the box will behave as a quantum dot. The formation of quantum dot active regions from self-organised island growth is widely reported in the art.
One driving force for the self-organised formation of islands during epitaxial growth can be the strain caused by there being a difference between lattice parameters that a deposited material would adopt when it is elastically relaxed and the equivalent lattice parameters in the substrate near to the growth surface which the deposited material adopts during epitaxial growth (the relevant lattice parameters are those lying in the plane parallel to the growth surface of the substrate). When there is such a lattice parameter mismatch between the deposited material and the substrate or underlayer over which it is deposited, a commonly observed behavior is that the deposited material initially forms a flat layer which covers the entire surface (this is referred to as two-dimensional (2D) growth) and then, as the deposition proceeds further, islands nucleate on the surface of the layer and the subsequently deposited material is accommodated into these islands, rather than into the flat layer (this is referred to as three-dimensional (3D) growth). The epitaxial growth of a layer which initially occurs in the 2D mode and then proceeds in the 3D island-growth mode is commonly referred to as Stranski-Krastanow growth, and will be referred to as SK growth hereafter. The flat layer which forms during the 2D growth stage is commonly referred to as a wetting layer. The thickness of the wetting layer at which the transition from 2D growth to 3D growth takes place is commonly referred to as the critical thickness of the wetting layer. The formation of islands by SK growth has been reported for a wide range of materials systems in the prior art.
The driving force for the transition from 2D growth to 3D growth depends on the mismatch between the lattice parameter of the deposited material and the lattice parameter of the substrate surface. The detailed properties of SK growth and the transition from 2D growth to 3D growth depend on the epitaxial deposition conditions and on the materials involved. However it is generally reported in the prior art for a wide range of materials systems that the critical thickness decreases as the lattice parameter mismatch between the underlayer and the deposited material increases.
The influence of the lattice parameter mismatch between the deposited material and the underlayer on the transition from 2D growth to 3D growth may be used to control the regions on a substrate within which islands are nucleated during SK growth. This effect is exploited in U.S. Pat. No. 5,614,435 in a method which results in preferred nucleation of islands only in pre-determined regions of the substrate. A suggested method for achieving this preferred nucleation is for the SK growth to take place on a substrate where the surface strain of the substrate varies between the regions where island formation is desired and the regions where no island formation is desired. This difference in surface strain means that there is a difference in the lattice parameter mismatch between the deposited material and the substrate surface in the two types of region. The difference in mismatch results in different critical thicknesses between the regions where island formation is desired and the regions where no island formation is desired during SK growth, and the substrate surface strains are chosen so that the critical thickness is larger in the regions where no islands are desired. Provided that the amount of material which is deposited during the SK growth exceeds the smaller critical thickness and does not exceed the larger critical thickness, islands are only formed in the desired regions. U.S. Pat. No. 6,583,436 describes similar control over the regions at which islands will be nucleated according to modulations in the surface strain of the substrate used for the SK layer growth.
A method which demonstrates the importance of the substrate surface lattice parameter in determining the properties of islands formed during SK growth is described in U.S. Pat. No. 6,507,042. It is described that the emission wavelength from light-emitting devices with quantum dot active regions formed using SK growth can be adjusted by selection of the surface lattice parameter of the substrate or buffer layer on which the SK growth is carried out. This selection of the surface lattice parameter of the substrate or buffer layer is achieved by selection of an appropriate composition for said substrate or buffer layer. Through selection of substrates with larger lattice parameters, active regions containing InxGa1−xAs quantum dots with emission wavelengths at longer wavelengths were formed. Although this substrate surface lattice parameter selection approach delivers a way of adjusting the emission wavelength from devices grown in separate epitaxial deposition layers, there is no facility to deliver control of quantum dot properties within a single layer.
An existing approach in the prior art which can deliver this desirable in-plane control of the properties of quantum dots formed from SK island growth is described by S. Mokkapati et al. in “Controlling the properties of InGaAs quantum dots by selective-area epitaxy”, Applied Physics Letters 86 113102 (2005). This method is commonly referred to as selective-area epitaxy. In this approach the substrate surface which is to be used for SK growth is patterned with masked regions. During the subsequent SK growth, the deposited material does not adhere to the masked regions and may migrate across the mask to the regions of the substrate surface which are not masked. A consequence of this migration from the masks is that the total amount of material which arrives in the regions between masks can depend on the size of the masked regions and the size of the regions in between the masks. Therefore, by use of different mask patterns in different regions of a substrate, the islands formed during SK growth can have different properties in different regions of the substrate. One significant disadvantage of the selective-area epitaxy method is that it relies on direct epitaxial growth onto the masked surface and therefore residual contamination at the surface caused during the fabrication of the mask can degrade the properties of the resulting quantum dots. A further significant limitation of the method is that a mask is deposited on the substrate surface each time a new SK growth is to be carried out and therefore the method is poorly suited to preparation of quantum dot active regions for light emitting devices where more than one layer of SK growth is often performed within each device structure so that the emission intensity from the device can be high.
JP-A-2003 309322 discloses preparing a substrate having a bulk InP layer over which are provided alternating regions of InP and GaInAsP. InAs quantum dots are then grown over the substrate.
Other related prior art includes U.S. Pat. Nos. 5,075,742, 5,952,680 and 6,445,009, and B. Damilano et al. “From visible to white light emission by GaN quantum dots on Si(111) substrate” Appl. Phys. Lett. 75 962 (1999).